The present invention relates to a shock absorber or hydraulic damper, and in particular to a shock absorber having a damping valve for controlling the flow of fluid between two fluid chambers in the shock absorber.
Known shock absorbers comprise an inner tube; an outer tube; a compensation chamber formed between the inner and outer tubes; a piston assembly sealably slidably mounted in the inner tube, the piston assembly separating a compression chamber from a rebound chamber within the inner tube; a rod guide positioned at one end of the inner and outer tubes; a piston rod connected to the piston assembly and extending through the rebound chamber and the rod guide; and a compensation valve assembly positioned at the other end of the inner tube. These known shock absorbers are commonly referred to as twin-tubed dampers. In these known arrangements, the piston assembly typically comprises a number of apertures normally closed by one of two valve elements (such as resilient discs). One valve element allows flow of fluid through some of the apertures from the rebound chamber to the compression chamber, and the other valve element allows flow of fluid through the remaining apertures during use of the shock absorber. Similarly the compensation valve assembly allows fluid flow between the compression chamber and the compensation chamber during use of the shock absorber. Bleed passages are also provided through the piston assembly and the compensation valve assembly to allow restricted flow of fluid during periods when the piston assembly is moving at a slow speed relative to the inner tube. This arrangement has the disadvantage that adjustment or setting of the damping characteristics of the suspension strut, especially at slow speeds is extremely difficult to control. Further, the apertures and bleed passages are susceptible to being easily blocked by dirt.